1. Field of the Invention
The present invention relates to an application for a communication system, and more particularly, to a rate adaptation method for a communication system.
2. Description of the Related Art
Communication systems, especially wireless communication systems, operate in a transmission environment that is constantly varying due to a multitude of factors, such as weather, transmission distance, velocity of transmitter or receiver, and noise. Therefore, the initial transmission rate of a communication system may not be suited for the transmission environment at a later time. If a higher transmission rate is acceptable for the varied transmission environment, then the communication system wastes available transmission resources. On the other hand, if the original transmission rate is not acceptable for the varied transmission environment, then the transmission throughput will drop tremendously. Therefore, there is a need for a communication system to adjust the transmission rate according to the varying transmission environment.
Most conventional rate adaptation methods update the transmission rate based on a transmitted probe signal or the signal quality of the received signal at the receiver within a fixed period. Auto rate fallback (ARF) and adaptive ARF (AARF) algorithms transmit probe packets to determine whether a higher transmission rate is achievable, and lower the transmission rate if transmission failure occurs successively. A sample rate algorithm transmits probe packets of different modulations and transmission rates periodically, and determines a transmission rate based on the receiving status of the transmitted probe packets. However, the aforementioned conventional rate adaptation methods all require additional probe packets such that the transmission throughput is reduced, and thus are not practical.
Onoe algorithm, adaptive multi-rate retry (AMRR) algorithm, Madiff algorithm and robust rate adaptation algorithm (RRAA) are examples of rate adaptation methods. Onoe algorithm raises the transmission rate when the packet error rate is below 10% a successive number of times, and lowers the transmission rate when the average packet retry rate is greater than 1. AMRR algorithm and Madiff algorithm adopt the thresholds of two packet correct rates to determine whether to raise or lower the transmission rate. RRAA algorithm determines transmission rate based on the ACK signals of transmitted signals and the packet correct rate. However, the aforementioned conventional rate adaptation methods all require waiting or measuring the transmission quality for a period of time. Madiff algorithm, AMRR algorithm and RRAA algorithm, for example, all require at least ten packets for the determination of transmission rate. Therefore, these rate adaptation methods converge too slowly.
In view of the disadvantages of the conventional rate adaptation methods, there is a need to design a rate adaptation method which not only would not reduce the transmission throughput, but also converges quickly to the optimum transmission rate.